Metformin is a worldwide used and effective antihyperglycemic agent. It is generally well tolerated but contraindicated in severe renal dysfunction. Metformin does not cause hypoglycaemia, however this case describes hypoglycaemia after an overdose. Another well-known, but rare and potentially life-threatening complication of metformin is the occurrence of LA, the so-called metformin-associated lactic acidosis (MALA). The severe consequences of an overdose of metformin, as well as the pathophysiologic mechanisms and the clinical consequences of LA in combination with severe hypoglycaemia are described in this report. Early recognition and swift clinical intervention is of paramount importance in case of auto-intoxication with metformin to prevent a lethal outcome.